Ordinary concrete consists of mineral aggregate (sand and gravel) bound together with Portland Cement, the latter hardening by reacting chemically with water to form a hydrated solid matrix integral with the aggregate. Polymer concrete also contains mineral aggregate and fillers but uses instead of Portland Cement various organic resins, which polymerize to bind the aggregate and form a solid matrix. The term "polymer concrete" as used here as well as in the technical literature does not include polymer-extended, ordinary concretes. The latter are aqueous systems, containing water-soluble resins or latexes, usually in conjunction with Portland Cement.
In contrast, polymer concretes do not ordinarily contain free water in their formulation. (See "Polymers in Concrete": Proceedings of the Second International Congress, Oct. 25-27, 1978, pages 1-4, University of Texas at Austin, 1978.) Hardening of polymer concrete generally involves organic reactions (polymerization, cross-linking) which solidify the resin and bind it to the aggregate and filler. These reactions are triggered by heat or by special chemical agents which are added to the resin. The overall process is termed "curing" or "setting". The advantages of polymer concrete include fast curing rates, much higher strengths than ordinary concrete and impermeability to water. While the above properties are important in determining areas of application of polymer concrete, this invention focuses on other properties: the dimensional changes (shrinkage or expansion) of the polymer concrete system as it hardens.
Polymer concretes and mortars have an inherent tendency to shrink as they cure. This is due to the polymerization and cross-linking reactions, which increase the density of the hardening resin. For example, polymerization of methylmethacrylate produces a shrinkage of up to 23%; polymerization and cross-linking of unsaturated polyester-styrene systems, a shrinkage of 8-12%.
In PCs prepared in accordance with prior art, the cure shrinkage of the overall systems is maintained at levels of 2% or less by the use of large amounts of inert fillers and aggregate (as high as 90% by weight of the total system). However, the concentration of shrinkage in the polymer matrix of these systems creates substantial local stresses, "setting stresses," which reduce the strength of the cured composite.
It is important to distinguish between cure shrinkage and setting stresses in polymer concrete. Both are due to shrinkage forces, generated during the polymerization or cross-linking of the resin, when hitherto secondary chemical bonds between organic molecules are transformed to primary (covalent) bonds which have much smaller interatomic distances. In polymerization systems, containing little or no mineral filler, these forces are accommodated by a volume reduction in the still-fluid resin during the pre-gel part of the cure; this would constitute the cure shrinkage. In PCs, however, where the particles of the filler and aggregate occupy about 70-75% of the total volume, these solid particles are nearly close packed. The liquid resin is mostly confined to the small spaces between particles which cannot change in volume. In these confined spaces the cure shrinkage forces are not relieved and give rise to local tensile stresses which increase progressively as the system hardens with cure. These are the setting stresses.
Both cure shrinkage and setting stresses are highly undesirable. Excessive shrinkage during cure tends to impair moldability, dimensional stability, and the appearance of the product. Setting stresses reduce the ultimate strength and significantly impair the creep resistance of polymer concrete. Severe shrinkage stresses can cause cracking in the polymer to a degree which may result in failure of the concrete structure.
The parent of this application, U.S. Ser. No. 198,620, discloses the use of small quantities of an expandable hydrated mineral additive in PCs to control cure shrinkage and eliminate setting stresses. Indeed, when added in sufficient quantity, these minerals produce an expandable PC. The minerals useful as additives to control cure shrinkage include those minerals having between about 10-25% of releasable water of hydration within their crystal lattices such as the montmorillonite group of minerals: montmorillonite (MMT), beidellite, montronite, saponite and hectorite.
These minerals may be used with most thermosetting polymer systems known in the art provided that the PC systems are formulated and processed so as to attain an internal temperature of 100.degree. C. or greater during the curing stage. The MMT, or similar mineral, is dispersed within the thermosetting resin in the form of fine particles and is coupled to the resin with small amounts of organo-silanes or similar coupling agents. When these compositions are cured at oven temperatures of about 60.degree.-80.degree. C., the polymerization exotherm raises the internal temperature of the curing system to about 120.degree.-140.degree. C. thereby releasing some of the hydration water from the MMT crystals in vapor form. However, the MMT particles containing these hydrated crystals are imbedded in and bound to the resin which at this point has polymerized to a highly viscous state. This encapulation of the MMT particles in viscous resin confines the generated water vapor within the MMT particles, causing these particles to swell to 3-4 times their original size. This swelling counteracts the cure shrinkage of the resin and minimizes setting stresses in the cured system without generating low-strength domains (such as bubbles) which would appear if the vapor were to escape from the MMT particles. The binding of MMT to the resin via silane coupling agents further enhances the integrity of the system. The end result is a most advantageous combination of cure shrinkage control (from zero shrinkage to a net expansion of a few percent) as well as strength enhancement of the cured polymer composite.
While the expandable PC of U.S. Ser. No. 198,620 presents an advance over the prior art, it requires that the internal temperature of the polymerizing system exceed 100.degree. C. during cure in order to trigger MMT dehydration. This requirement limits application of the invention to oven-cured systems, thus precluding its use in large field-erected polymer concrete structures (which are cured at ambient temperatures) and severely curtailing its use with systems based on methyl methacrylate, a monomer that is too volatile to cure at 100.degree. C.
The instant invention presents a significant advance: it overcomes this cure temperature limitation by replacing the hydration water of the naturally-occurring mineral additive, such as MMT with one or several polar liquids of high volatility such as ammonia, aliphatic amines, such as methylamine, dimethylamine, methylethylamine, alcohols, such as methanol, and ketones, such as acetone. Adding these substituted MMT's in suitable proportions to thermosetting resin systems makes possible the production at ambient cure temperatures of PCs having the cure shrinkage control and strength enhancement formerly achieved with the natural MMT only at cures above 100.degree. C.